1. Field of the Invention
The present invention relates to naturally occurring truncated forms of Hepatocyte Growth Factor (HGF), which are encoded by alternative HGF mRNA transcripts. In particular, the present invention relates to a small HGF variant, HGF/NK2, which is a competitive antagonist of HGF, and HGF/NK1, which is a partial agonist of HGF.
2. Background Information
Hepatocyte growth factor has hormone-like activity and is released in response to partial hepatectomy and liver injury and is presumed to be an important mediator of liver regeneration (Nakamura et al., FEBS Lett. 224: 311 (1987); Gohda et al., J. Clin. Invest. 81: 414-419 (1988); R. Zarnegar and G. Michaelopoulous, Cancer Research 49: 3314-3320 (1989)). Its ubiquitous expression by stromal fibroblasts and demonstrated ability to stimulate DNA synthesis in melanocytes and endothelial cells as well as epithelial cells suggest that this factor plays a role in paracrine regulation of cell growth as well (Rubin et al., Proc. Natl. Acad. Sci. USA 88: 415 (1991)). Reports showing that scatter factor has high amino acid sequence identity to HGF over restricted regions, suggested that HGF may also be involved in modulating cell-cell interactions and migration (E. Gherardi and M. Stoker, Nature 346: 288 (1990); Weidner et al., J. Cell Biology 111: 2097-2108 (1990)). This has been borne out by subsequent studies that verify the identity of scatter factor and HGF.
Structurally, HGF resembles plasminogen in that it possesses characteristic kringle domains (Patthy et al., FEBS Lett. 171: 131-136 (1984)) and a serine protease-like domain (Miyazawa et al., Biochem. Biophys. Res. Commun., 163: 967-973 (1989); Nakamura et al., Nature 342: 440-443 (1989)). Like plasminogen, HGF can be processed by proteolytic cleavage, generating a heterodimeric molecule comprised of a heavy- and light-chain covalently linked by disulfide bonds (Nakamura et al., Nature 342: 440-443 (1989) and Miyazawa et al., Biochem. Biophys. Res. Commun. 163: 967-973 (1989)) The possibility that its actions might be mediated by a receptor tyrosine kinase was suggested by its rapid stimulation of tyrosine phosphorylation of cellular proteins in target cells (Rubin et al., Proc. Natl. Acad. Sci. U.S.A. 88: 415 (1991)). Recent studies have directly identified the HGF receptor as the c-met protooncogene product (Bottaro et al., Science 251: 802 (1991)), whose structure resembles that of a membrane-spanning tyrosine kinase (Park et al., Proc. Natl. Acad. Sci. USA 84: 6379-6383 (1987); Chan et al., Oncogene 2: 593-599 (1988)).
There is accumulating evidence that the positive effects of growth factors on cell proliferation can be counteracted at a variety of levels both intracellularly (Moses et al., Cell 63: 245-247 (1990) and at the cell surface (Hannum et al., Science 343: 336-340 (1990), Eisenberg, et al., Nature 343: 341-346 (1990); Carter et al., Nature 344: 633-637 (1990)). Thus, the potential exists to find an antagonist to HGF which would negatively regulate the growth factor's proliferation effects. The invention described herein relates to small HGF variants and their corresponding transcripts. Characterization of one of these HGF variants, HGF/NK2, has revealed that it is a competitive antagonist of HGF action and thus establishes a novel regulatory mechanism whereby the same gene encodes both an agonist and antagonist of growth factor action. Characterization of another HGF variant, HGF/NK1, revealed that it is a partial agonist of HGF in vitro.